Does the presence of congenital hernias in children predispose to hernias in adulthood especially since the repair is only a herniotomy but no repair of the posterior wall of the inguinal canal?

Unknown. As you said there is no repair of the posterior (floor) of the inguinal canal. However they are separate entities with different causes and one should not lead to the other or preclude it. Finally on occasion wih long standing child type hernias can lead to weak floors and this is usually reinforced if discovered at the first operation.
No. Congenital inguinal hernias in children have a different cause. They result from non closure of an opening that allows organs to pass in utero. This opening then closes prior to birth. A simple closure is required. In adults there is muscle destruction and replacement with scar tissue. This type of hernia requires a more extensive repair, often with mesh or a prosthetic.